EXCEED THE SPACE PROVIDED. The overall objective of the research proposed in this grant application is to continue our analysis the molecular pathways that regulate B lineage commitment and maturation. Our studies would be focused on the regulation and function of a subset of helix-loop-helix proteins, named E-proteins, in B-lineage development. E-proteins, E12,E47, HEB and E2-2, function at multiple stages thorughout B cell development. Specifically, E-proteins act:(1) to activateB- lineage specific gene expression, (2)to control Ig light chain gene rearrangement, (3)toregulate receptor editing, (4) to enforce the developmental checkpoint at the immature-B cell stage, (5)to regulate marginal zone versus follicular zone B cell development and (6) to induce AID gene expression. During the next grant cycle we would continue these studies. In particular we would investigate the relationship involving E2A, EBF and Pax-5 in early B lineage development. We would examine how E2Aproteins activateand repress transcription and how they regulate Ig light chain gene rearrangement. We have recently demonstrated that theE2A proteins have the ability to repress transcription by recruitment of a transcriptional co-repressor, named ETO-2. We would examine the function of ETO-2 during B cell development and how the activity of ETO-2 is regulated. We have also recently determined using mass spectrometry two serine residues in E12and E47that are modifiedby phosphorylation and one lysine residue modified by methylation. We would introduce mutations in these residues and examine their impact on the activity of E2A both in vitro and in the mouse germ-line. We would further characterize the role of E-proteins in the regulation AID transcription. Overall the dissection of the roles of E2A in B cell development should help to clarify the mechanism by how E-proteins regulate Blineage maturation.